Well, it wasn't really, it was only formally wealthy and really in decline.

The simple answer today for why America looks wealthy on paper that our currency is chronically over-valued from our historical position as the last super power. From this, all the things we produce seem more valuable than they are. Of course, we aren't actually selling these things to that many people in other countries. Instead we coast on the credit China is willing to conveniently advance us to buy their goods. We're "so productive" we no longer need to produce ... that much.

The world's overall currency and trade relations are amazing unbalanced with both US and Chinese policy makers apparently willing to feed the illusionary value of the dollar for as long as it suits their mutual policy interests.

Silicon Valley is a huge source of innovation but it's one of last things of value to the world that the US has. There are other things on that list but the list hasn't gotten longer for a while now.

Also, US government spending is on-par with Europe (~33%+ of GDP!) but the US somehow provide little to no "safety net" to its citizens. The US still a large manufacturer - it's just that a large portion of it's manufacturing goes directly to the defense sector.